1. Field of the Invention
The present invention relates to molded polymer articles having a hydrophilic surface, a process for producing the same and a mold used therefor. More specifically, the present invention relates to molded polymer articles having on the surface thereof a hydrophilic polymer layer which is excellent in the adhesiveness to the main molded body and in durability, a process for producing the same and a mold used therefor.
The molded polymer articles of the present invention can be effectively used, utilizing the above characteristics, for a wide variety of end-uses including medical products such as contact lens, which should be or are desired to be excellent in hydrophilic property and biocompatibility.
The process according to the present invention can produce very smoothly the above molded polymer articles having a hydrophilic surface, provided with the excellent properties.
2. Description of the Prior Art
Modification of the surface of a molded polymer article is widely practiced, to provide the article with a new function which the polymer constituting the main body of the article does not originally possess. In particular, in the field of medical products, various surface modification techniques have been attempted to improve the biocompatibility of the polymer used to human body. Among such medical products, lenses for human eyes, such as contact lenses and intraocular lenses, are made principally of silicone resin and acrylic resins containing silicon and/or fluorine atom. These materials are highly hydrophobic and tend to cause troubles such as lens sticking to cornea, clouding of lens and poor comfort during wear. For contact lenses and similar items obtained from these materials, various attempts have therefore been made to treat the surface to provide it with hydrophilic property, thereby eliminating the disadvantages of lens sticking to cornea and clouding of lens, and improving the comfort during wear.
The processes for rendering hydrophilic the surface of contact lenses or intraocular lenses are roughly classified into: (A) one which comprises at first preparing a lens and then treating its surface to provide hydrophilic property; (B) one which comprises curing and shaping a previously produced polymer into a lens and, at the same time, rendering the surface hydrophilic; and (C) one which comprises molding a polymerizable monomer into a lens and, at the same time, rendering hydrophilic the surface of the resulting article.
The above process (A), producing a lens and then rendering its surface hydrophilic includes: (1) a process which comprises treating a lens with an aqueous alkali solution or hot salt water; (2) one which comprises subjecting a contact lens to discharge treatment in a rarefied gas (see Japanese Patent Publication No. 49288/1980); (3) one which comprises graft polymerizing onto the surface of a contact lens a specific hydrophilic monomer having a phosphate group or a vinyl monomer having a specific phosphatide-like structure (see Japanese Patent Application Laid-open Nos. 122779/1994 and 72430/1995); and (4) one which comprises placing a contact lens under an atmosphere containing a monomer having hydrophilic group and polymerizing the monomer by electrodeless glow discharge to form and bond a film of the resulting hydrophilic polymer onto the surface of the contact lens (see Japanese Patent Application Laid-open No. 83642/1978).
However, the above processes (1) and (2) have the disadvantages that the hydrophilic property of the hydrophilic surface formed on the surface of the lens does not last for a long period of time and is lost in a short period. With the above process (3), insufficient bonding force between the polymer constituting the contact lens main body and the hydrophilic polymer formed by graft polymerization weakens the durability of the latter, so that, in this case also, the hydrophilic property is lost in a short period of time. Likewise, with the above process (4), the hydrophilic polymer film formed on the surface of the contact lens adheres to the lens main body insufficiently and tends to be separated therefrom, so that the hydrophilic property of the surface does not last for a long period of time and tends to be lost when external forces such as friction are applied to the surface.
The above known process (B), which comprises curing and shaping into a lens a previously produced polymer and, simultaneously therewith rendering the surface hydrophilic includes: (5) a process which comprises covering the surface of a previously produced polysiloxane with a radical polymerizable monomer capable of giving a hydrophilic polymer and then irradiating the monomer and polysiloxane with ionizing radiation, to crosslink and shape into a lens the polysiloxane and, at the same time, graft-polymerize the monomer on the surface of the polysiloxane, thereby producing a contact lens covered with the hydrophilic polymer (see Japanese Patent Application Laid-open No. 775/1973, U.S. Pat. No. 4,099,859).
However, this process (5) yields insufficient adhesion on graft polymerization of the radical polymerizable monomer to the polysiloxane, whereby the hydrophilic polymer formed from the monomer becomes not durable and tends to be separated from the polysiloxane. This process is thus not usable for practical purposes.
The above known process (C), which comprises molding a polymerizable monomer into a lens and, simultaneously therewith, rendering the surface hydrophilic includes: (6) one which comprises covering the surface of a mold with a film of a hydrophilic polymer having copolymerizable functional groups, injecting into the mold a monomer or mixtures thereof required for forming a hydrophobic polymer and copolymerizing the monomer or mixtures thereof with the functional groups present in the film, thereby chemically bonding the film of the hydrophilic polymer to the hydrophobic polymer to obtain a contact lens having on the surface thereof a hydrophilic polymer film (see Japanese Patent Application Laid-open No. 124523/1990) and (7) one which comprises copolymerizing a monomer mixture principally comprising a (meth)acrylic acid ester and a crosslinkable monomer in a lens mold the molding surface of which has been covered with a polymer having a release property and hydrophilic property, to produce a copolymer having a contact lens shape and then plasma-treating the surface of the lens (see Japanese Patent Application Laid-open No. 266443/1995).
However, with the above process (6), it is difficult to form a uniform layer of the hydrophilic polymer on the surface of the molded hydrophobic polymer. Furthermore, the formed hydrophilic polymer bonds only insufficiently to the molded hydrophobic polymer. As a result, the product has poor durability and permits its wettability (hydrophilic property) to decrease by such a slight friction as by hand.
With the above process (7), which comprises covering the molding surface of a mold with a hydrophilic polymer having no copolymerizable functional group, virtually no chemical bond is formed, on the polymerization, at the adhesion interface between the hydrophilic polymer layer and the lens substrate and, on the succeeding plasma treatment, the activated points generated on the surface of the hydrophilic polymer layer can hardly react and bond with the underlying lens substrate. Consequently, the products obtained by this process (7) have poor adhesiveness between the hydrophilic polymer layer and the lens substrate, thereby tending to permit the hydrophilic polymer layer to be separated from the lens substrate, and thus have poor durability and are not usable in practice.
There has been proposed a process (8), belonging to a category separate from the above processes, which comprises, on producing molded articles in a mold by copolymerizing a monomer containing at least one silicone segment and at least one hydrophilic monomer, using the mold prepared from a copolymer of an .alpha.,.beta.-olefin-type unsaturated mononitrile and at least one comonomer capable of enhancing the processability in the melted state, to provide the obtained molded articles with hydrophilic property (see Japanese Patent Tokuhyouhei 6-503276). With this process (8), it is considered, not so clearly though, that the hydrophilic monomer present near the mold surface migrates by diffusion, by the initial stage of polymerization at the latest, to be adsorbed on the nitrile groups present on the surface of the mold, so that the hydrophilic monomer is condensed on the mold surface, thereby forming the hydrophilic surface of the resulting contact lens.
However, in the products, such as contact lens, obtained by this process (8), the units from the hydrophilic monomer used are present inside the products, thereby impairing the performance originally possessed by the substrate silicone resin. As a result, in particular, the oxygen permeability of the contact lens and like items decrease. Furthermore, this process (8) has the disadvantage that the hydrophilic monomer usable for producing the molded articles is limited to one which is compatible with the counterpart monomer having at least one silicone segment.